An approach is developed for applying diffusion Monte Carlo (DMC) techniques to problems that involve two or more coupled potential energy surfaces. This technique combines surface hopping with traditional DMC approaches. It is applied to linear chains of HF when at least one of the HF monomers is excited to a low-lying vibrational state, v = 1 or v = 2. In the case of (HF)(2) the errors in the energies, calculated using DMC, are smaller than the statistical uncertainty. In this case, the vibrational excitation is localized, but, as the number of HF monomers is increased to 10, the excitation becomes delocalized.